The invention relates to electronic audio signal systems and devices. The invention also relates to digital communications.
Data compression is extremely important to the music industry. In digital audio signal systems, digital samples of sound are stored on a Compact Disk Read Only Memory (CD ROM). Fidelity of the sound is proportional to the rate at which the sounds are sampled (the sampling rate) and the number of bits comprising each sample. An audio signal sampled 22,000 times per second (22 kHz) by a 16-bit analog-to-digital converter (ADC) is of far higher fidelity than an audio signal sampled at 11 kHz by an 8-bit ADC. An audio signal sampled at 44 kHz by a 24-bit ADC is of even higher fidelity. However, the 44 kHz, 24 bit sampling produces three times as much data as the 22 kHz, 16-bit sampling and twelve times as much data as the 11 kHz, 8-bit sampling. This is where data compression is so important. The data compression reduces the amount of data stored on the CD ROM, but maintains the fidelity of the sound. Data compression allows an audio signal sampled at 44 kHz by a 24-bit ADC to be stored economically on a CD ROM.
Data compression is also important to the television industry, especially with the emergence of direct broadcast television. In a direct broadcast system, digital signals of near-perfect video images and audio waveforms are encoded according to a known standard, transmitted to a satellite orbiting the earth, and relayed by the satellite on the Ku band to any home equipped with a small dish antenna and a receiver unit. Data compression reduces the amount of video and audio data that must be transmitted.
One compression standard becoming widely used is the MPEG standard. MPEG was established by the Moving Pictures Experts Group of the International Standardization Organization to specify a format for the encoding of compressed full-motion video and audio. MPEG audio compression produces CD quality audio at very high compression rates.
When playing any digitally-encoded source, such as a CD player, it sometimes becomes desirable to mute the audio output. One technique known as "hard-muting" is performed by abruptly terminating the audio output. A Mute signal is generated and AND'ed with the audio output (see FIGS. 1a and 1b). The hard mute is akin to pulling the plug on the CD player. The problem with this technique is that it causes a discontinuity in the audio output (in FIG. 1b, the discontinuity appears as a square edge of the audio waveform). The discontinuity produces a very audible and very loud and undesirable "thump" that is potentially damaging to human ears and speaker woofers. In addition, the discontinuity produces very high-amplitude, high frequencies (ringing) that can damage the speaker's tweeters. The problem is repeated when the hard-mute is released. The audio signal is abruptly resumed, and another discontinuity is created.
A second technique for muting the audio output is performed by freezing and repeating the last sample on the audio output (see FIG. 1c). This technique avoids the abrupt termination of the audio output and, therefore, the first discontinuity. However, when the mute is released, the audio output is abruptly resumed, and a discontinuity is created. Thump|
A more sophisticated (third) muting technique uses scale factors to perform a quick fade on the audio output (see FIG. 1d). Prior to output, an audio signal is multiplied by a scale factor ranging between 0 (no volume) and 1 (normal volume). By ramping the scale factor between 1 to 0, the audio signal is scaled down and up without discontinuities. This technique is similar to quickly turning the volume down and up. Because there are no discontinuities, this technique is more pleasing to the ear and less damaging to the speakers. One disadvantage of this technique, however, is the need for additional hardware for generating the scale factors and multiplying the audio signal with the scale factors. Another disadvantage is the difficulty of concealing or limiting errors encountered when corrupted data read from the CD ROM is decoded by the CD player. Such corrupted data creates an error factor that propagates to the audio output. The third technique does not ramp down the audio output both slowly enough to avoid ringing and quickly enough to remove from the audio output errors caused by the improperly coded vectors. In fact, erroneous data will always sneak through and most likely be audible, regardless of the rate at which the scale factor is ramped.